1. Field of the Invention
The invention relates generally to the field of air-conditioning/heating systems, and more specifically to such systems having multiple capacity.
2. Description of the Prior Art
In typical rapid transit systems using railway cars, buses and the like, the air-conditioning/heating systems are normally designed to have sufficient capacity, that is, heating and cooling power, to handle rush-hour or peak-load heating and cooling demands. During off-peak hours, however, the capacity of the system must be reduced to avoid excess heating and cooling.
A conventional air-conditioning system includes an evaporator with a blower, a compressor, a condenser and a storage tank for a refrigerant fluid. The refrigerant is cyclically pumped from the tank through the evaporator, and the compressor to the condenser, and back to the storage tank. In the evaporator, air is blown through the evaporator coils. The refrigerant pumped through the coils picks up heat from the air, thereby cooling the air. The refrigerant is then cooled in the condenser and returned to the refrigerant storage tank.
In the prior art, when it was desired to change the cooling capacity of the air-conditioning system, a two-feed evaporator was provided having two independent evaporator coils in parallel, one of which could be closed off to reduce the cooling capacity of the system. Also, a compressor was provided, normally of the multiple-piston type, which could be unloaded by closing off certain of the cylinders to reduce the pumping speed of the refrigerant through the compressor.
Such prior art air-conditioning/heating systems, however, use excessive amounts of energy even in the reduced capacity mode. When the speed of the compressor motor is lowered, by varying the series resistance, no significant reduction in energy consumption is realized at the slower speed. Providing the evaporator with a second feed is also expensive and wasteful, because the mechanism provided for the second feed is used only for a small percentage of the time the system is in use.
The present invention provides a dual capacity air-conditioning/heating system having a single circuit evaporator and a two-speed compressor motor, the two speeds being achieved by changing the amount of resistance in series with the shunt field coil, of the motor, thereby changing the current through the shunt field. This achieves a significant savings in the energy consumed during low-speed operation. The reduction in speed of the compressor motor also achieves a significant reduction in the noise from the system. Providing the evaporator with a single feed achieves a significant savings in manufacturing costs of the air-conditioning/heating system.